1. Field of the Invention
The invention relates to a conduit joint and more particularly to a joint wherein conduit sections each having one end threaded are joined by a coupling ring which permits the sections to swivel slightly with respect to one another during coupling and which allows the sections to be disassembled without being rotated and without axial longitudinal displacement.
2. Description of the Prior Art
Until very recently, there were on the market only two types of steel conduit. One of these was denominated as rigid pipe and the other as light walled conduit.
Rigid pipe has been available for over a hundred years. It is a thick walled pipe that is conventionally used for the transmission of water and gas. It is used also for sheathing electric wires in buildings where it frequently is embedded in concrete. As a sheathing for electrical wires, rigid pipe is used where there are high strength requirements or where the pipe is subjected to heavy or sudden shock loads. Still further, rigid pipe is used for drilling oil wells, for oil casings, and for encasing electric lines which are carried across bridges.
This type of pipe comes in sections of various lengths depending upon the trade in which it is employed but usually is from ten to twenty feet long.
Rigid pipe sections are connected by screw couplings. Both ends of a pipe section have tapered male threads formed thereon. These threads are V-shaped to standardize dimensions having a slightly flattened crest and a slightly flattened base. Both ends of a screw coupling have a tapered female pipe thread which mesh with a male pipe thread. The mesh is quite good and because the threads are tapered, when they are joined, they make a joint that is essentially hermetic. The joint does not require sealing means such as mastic or pipe compounds, although sealing means optionally may be employed.
There are certain disadvantages associated with the use of rigid pipe as for example when it is to act as a conduit for insulated electric wires. As a result of the pipe having thick walls, each section is expensive inasmuch as a large amount of steel is required. Also, each section is difficult to handle because of its weight. Each pipe section must be rotated in the field when it is connected to another length of pipe. The weight of the pipe section makes the rotation difficult. Another drawback is that if a section of pipe or a fitting is damaged, the pipe or fitting has to be replaced either by being cut and removed or by unscrewing the entire layout piece by piece until the damaged section is removed.
Moreover, since the screw coupling must mesh with male threads on adjacent pipe sections, the sections must be in perfect axial alignment in order for the threads to mesh properly. As is often the case in field installations where replacements must be made to existing pipe systems, the pipe sections are not in perfect alignment and problems result in assembling the sections.
The other kind of conduit which has been developed for sheathing electrical cables is light walled conduit. Light walled conduit was designed solely for the purpose of encasing electric cables to protect them from damage. Because it does not have to withstand large stresses, it is thin walled. An immediately obvious advantage of light walled conduit is its light weight and hence lower material cost as compared to rigid pipe.
An obvious disadvantage of light walled conduit is that it cannot be used in instances where it might be subjected to heavy loads. In addition, because of its thin wall, this type of conduit cannot be threaded. Compression couplings are most often used to join sections of light walled conduit. The ensuing joint is not able to withstand substantial internal pressure or tensile stress. The advantage of compression couplings over the screw couplings used in connection with the rigid pipe is that if any section of conduit or any other fitting is damaged, the coupling can be opened and the damaged section or fitting can be pulled out transversely without removing sections or fittings that were not damaged.
Recently, due to the scarcity (and consequently resulting higher price) of steel, a medium walled pipe known as utility pipe or intermediate metal conduit was developed having a wall thickness less than the wall thickness of the rigid pipe but greater than the wall thickness of the light walled conduit. The intermediate metal conduit is heavy enough to take intensive loads and can be hermetically sealed at the joints.
Intermediate metal conduit has used screw couplings for the joints, causing the same problems to arise in connection with this conduit as with the rigid pipe, namely, the necessity of rotating conduit sections to make up joints or the necessity of unscrewing the sections one after another from an end of a run of pipe when an intermediate section or fitting has to be removed. Further, the axial alignment problem inherent in the rigid pipe system is also present.
It is towards elimination of these and other problems that the present invention is directed.